Diffusion transfer photographic processes, in which silver salts like silver halides are utilized, are well known.
In one type of such process, a light-sensitive element containing an optically exposed silver halide photographic emulsion is brought into face-to-face contact with an image-receiving element containing silver-depositing nuclei, and an alkaline processing solution containing a silver halide solvent is applied in a layer form between these two elements in the presence of a developing agent, whereby development-processing is effected to directly form a positive silver image on the image-receiving element.
More specifically, the unexposed portion of the silver halide emulsion in a light-sensitive element is dissolved in a silver halide solvent in the form of silver ion complex, and then eluted in an alkaline processing solution. Through the elution, the silver ions are transferred into an image-receiving element, and silver-depositing nuclei present in the image-receiving element act on the silver ions to precipitate them as a sliver image, thus forming a direct positive image.
However, the silver images obtained in an image-receiving element by the above-described process have a defect in that they tend to undergo a change in color or discoloration upon storage.
With the intention of obviating such a defect, methods of coating a water-soluble polymer solution containing an alkali neutralizing component on the surface of silver image obtained have been proposed, e.g., in Japanese Patent Publication No. 5392/71, U.S. Pat. No. 3,533,789, and British Pat. No. 1,164,642. However, these methods require a long time for the surface of the polymer coat to dry completely. When the polymer coat is in an undried condition, the surface thereof is so sticky that the prints cannot be placed one upon another. In addition, the formation of fingerprints and the adhesion of dust on the surface of the polymer coat is a frequent problem. Moreover, it is troublesome to require such an additional step of coating such a solution on silver images.
Japanese Patent Publication No. 44418/81 discloses an image-receiving element which comprises a support having provided thereon (I) a layer made up of cellulose ester, polyvinyl ester or polyvinyl acetal, which can be hydrolyzed in water and becomes alkali-penetrable by hydrolysis, and containing a diffusible compound capable of causing a change in a property of silver image, and (II) a regenerated cellulose layer containing silver-deposited nuclei, in the order listed. Therein, organic mercapto compounds are described as the compound which is diffusible and that can cause a change in a property of silver image.
On the other hand, Japanese Patent Publication Nos. 21140/81 and 500431/81 disclose that discoloration of silver image can be prevented by making noble metal compounds act on the silver image.
However, such organic mercapto compounds and noble metal compounds must be incorporated in large amounts in order to achieve the desired protection of silver image, and furthermore, they suffer from the defects that they cause stain, have undesirable influences on the image-forming speed, and so on.